Intraruminal doses of tryptophan cause interstitial pulmonary edema and emphysema in cattle. Ruminal organisms convert L-tryptophan to indoleacetic acid and then to 3-methylindole. Intraruminal doses of 3-methylindole cause pulmonary edema and emphysema in cattle and goats and it has been identified as the causative agent in tryptophan-induced pulmonary disease. Pulmonary lesions are related to the concentration and duration of 3-methylindole in the plasma of cows given intraruminal doses of tryptophan or 3-methyl-indole and intravenous doses of 3-methylindole. Current experiments include determination of lung concentrations of 3-methylindole associated with pulmonary lesions and inhibition of 3MI formation in the rumen. Attempts are being made to isolate the ruminal organism responsible for the formation of 3MI and identify its metabolic characteristics. Major emphasis is being given to the mechanism of action and metabolism of 3-methylindole in animals. Specific effects of 3-methylindole on artificial membranes, erythrocyte ghosts, and lung lavage cells are being investigated.